Gear machine tools have conventionally been provided as machines for gear machining of a workpiece, or a work gear, with a rotary tool. Among such gear machine tools, there are ones, such as hobbing machines and gear grinding machines, configured to perform generating machining on a workpiece with a rotary tool such as a hob cutter or a threaded grinding wheel. To perform such gear machining, the rotary tool and the workpiece are rotated about their respective axes, and from this state, the rotary tool is caused to cut into the workpiece. As a result, portions of the outer circumference of the workpiece are ground by a cutting part of the rotary tool, so that a tooth profile is created in the workpiece.
Here, the rotation of the workpiece about its axis is done by rotating a rotary table, on which the workpiece is mounted, about its axis. In other words, the workpiece must be mounted on the rotary table coaxially with the rotary table.
In general, a mount jig is mounted on the rotary table, and the workpiece is mounted on the rotary table through this mount jig. It is, however, difficult to mount the workpiece accurately coaxially with the rotary table due to factors such as machining errors and assembling errors in these components. Such centering of the workpiece is even more difficult particularly in a case of a large-sized workpiece measuring several meters in diameter and weighing several tons.
When the workpiece is mounted eccentrically on the rotary table, there occur not only an error in the cutting depth (in the radial direction) but also an error in the cumulative pitch (in the circumferential direction) of the gear. Correcting only the cutting depth is easy, but correcting the cumulative pitch in addition to the cutting depth is not easy because it complicates calculation formulae.
In this respect, gear machining methods have conventionally been provided which allow gear machining of a workpiece mounted eccentrically. Such a gear machining method is disclosed in Patent Document 1, for example.